1. Field of the Invention
The present invention relates generally to adjusting the air/fuel ratio in the cylinders of an internal combustion engine to control automotive emissions. More particularly, the present invention relates to a method and system for adjusting the air/fuel ratio in the cylinders based on the amount of oxidants stored in the catalytic converter.
2. Background of the Invention
To minimize the amount of emissions exhausted into the atmosphere, modern automotive vehicles generally include one or more catalytic converters, or emission control devices, in the exhaust system of the vehicle. These emission control devices store oxygen and NOx (collectively, oxidants) from the vehicle exhaust stream when the engine is operated with a relatively lean air/fuel ratio. On the other hand, when the engine is operated with a relatively rich air/fuel ratio, they release the stored oxygen and NOx, which then react with the HC and CO produced by the engine. In this way, the emission of both NOx and hydrocarbons (HC and CO) into the atmosphere is minimized.
The inventors have recognized a disadvantage with conventional air-fuel ratio control systems. In particular, the inventors have recognized that these systems attempt to maintain the engine at stoichiometry (or another desired air-fuel ratio). However, this has the disadvantage that engine air-fuel control is decoupled from the state of oxidant storage of the emission control device. The convention system relies on air-fuel feedback to compensate for this oversight.
The inventors herein have recognized that these known methods of adjusting cylinder air/fuel ratios, while effective, can be improved. In particular, the inventors have recognized that using the conventional air-fuel ratio control strategies tends to be reactionary in nature. That is, the cylinder air/fuel ratio tends to be adjusted more rich only after the exhaust stream oxygen sensors detect a NOx breakthrough. Similarly, the cylinder air/fuel ratio tends to be adjusted more lean only after the exhaust stream oxygen sensors detect hydrocarbon breakthrough. Further still, the inventors have recognized that this conventional method decoupled air-fuel ratio control from the state of the catalyst in terms of the oxidant amount stored in the catalyst.
The above disadvantage is overcome by a method for controlling an engine coupled to an emission control device. The method comprises determining an amount of oxidants stored in the emission control device; calculating a set-point amount of oxidants stored in the emission control device; and adjusting a fuel injection amount into the engine so that said amount of oxidants stored in the emission control device approaches said set-point amount of oxidants stored in the emission control device based on an error between said amount of stored oxidants and said set-point.
By adjusting fuel injection to control the stored oxidant level to a desired set-point, it possible to couple air-fuel ratio control to the oxidant storage level of the catalyst. In particular, this provides, for example, the opportunity to provide varying amounts, depending on operating conditions, to both:
(1) retain addition oxidants in the event the engine air-fuel ratio is inadvertently lean of stoichiometry; and
(2) reduce addition HC or CO in the event the engine air-fuel ratio is inadvertently rich of stoichiometry.
Thus, in one aspect of the invention, where the set-point is based on operating conditions, is possible to pro-actively prevent breakthrough of hydrocarbons, where such breakthrough normally would occur. Alternatively, it is possible to pro-actively prevent breakthrough of oxidants, where such breakthrough normally would occur. Rather than controlling the engine air/fuel ratio around stoichiometry per se, as in the prior art, the present invention controls the engine air/fuel ratio to maintain a certain amount of oxidants stored in the catalytic converter.
That is, in one example, the parameter upon which the controller calculates fuel adjustments is the difference between the actual amount of oxidants stored in the catalyst and a target amount of oxidants, i.e., the oxidant set point. Generally, if the actual amount of oxidants stored in the catalyst at a given time is greater than the oxidant set point, then the controller adjusts the engine air/fuel ratio more rich to produce hydrocarbons and release some of the oxidants from the catalyst. On the other hand, if the actual amount of oxidants stored in the catalyst at a given time is less than the oxidant set point, then the controller adjusts the engine air/fuel ratio more lean to produce additional NOx and replenish the relatively low amount of oxidants stored in the catalyst.
An advantage of the above aspect of the invention is improved overall catalyst efficiency and reduced emissions.
Also note, there are various methods that can be used to determine the oxidant set point. For example, the oxidant set point may be a constant value that is pre-determined at the time of manufacture. Alternatively, the oxidant set point may be a dynamically determined value that is calculated at various times during engine operation. In the latter method, an oxidant set point can be determined based on engine load, engine speed, and vehicle speed. Further, the oxidant set point can be adjusted based upon a prediction of imminent engine operating conditions. Finally, the oxidant set point can be modulated to increase catalytic conversion efficiency.